OsmoTRX is a software-defined radio transceiver that implements the Layer 1 physical layer of a BTS comprising the following 3GPP specifications:

TS 05.01 "Physical layer on the radio path"

TS 05.02 "Multiplexing and Multiple Access on the Radio Path"

TS 05.04 "Modulation"

TS 05.10 "Radio subsystem synchronization"

OsmoTRX is based on the transceiver code from the OpenBTS project, but setup to operate independently with the purpose of using with non-OpenBTS software and projects, while still maintaining backwards compatibility with OpenBTS. Currently there are numerous features contained in OsmoTRX that extend the functionality of the OpenBTS transceiver. These features include enhanced support for various embedded platforms - notably ARM - and dual channel diversity support for the Fairwaves umtrx.

OsmoTRX has been tested on the multiple embedded platforms representing a wide range of device types. Low cost ARM devices are generally limited by memory and I/O as much CPU utilization.

Running a full or near full ARFCN configuration (7 simultaneous TCH channels with Combination V) may require running the GSM stack remotely, which can be configured at runtime on the command line. This limitation appears to be scheduling related more so than lack of CPU resources, and may be resolved at a later time.

SSE3 is present in the majority of Intel processors since later versions of the Pentium 4 architecture and is also present on low power Atom processors. Support is automatically detected at build time. For additional performance information, please see the performance and benchmarks section.

ARM Support

NEON

NEON-VFPv4

OsmoTRX runs on a variety of ARM processors with and without NEON coprocessors. Like SSE on Intel processors, NEON provides acceleration with SIMD vectorized instructions.

NEON support must be enabled by the user at build time. For additional information, please see the configuration and performance and benchmarks sections.

Dual Channel (UmTRX and B210)

Two dual channel modes are available: standard dual channel mode and diversity. In standard dual channel mode, each RFpath of the dual channel device supports a different ARFCN. Each path operates independently and operates similarly to two separate devices. GSM channel capacity in this mode is doubled. This option can be configured at run time from the command line.

Dual Channel Diversity (UmTRX, experimental)

Diversity mode is similar to the standard dual channel mode except each antenna supports both ARFCN channels. In this case, the receiver sample bandwidth is widened to handle both ARFCN's and subsequently converted and demultiplexed into separate sample streams. Each GSM receive path is fed dual signals, where antenna selection diversity is performed by taking the stronger signal on a burst-by-burst basis. This diversity setup improves uplink reception performance in multipath fading environments.

Limitations are increased CPU utilization and that ARFCN spacing is restricted (currently at 400 kHz) by the receiver sampling bandwidth. Setting the ARFCN spacing beyond the sampling limit will disable the diversity path and operate in standard dual channel mode. This options can be configured at run time from the command line.

Uplink Burst Detection

OsmoTRX utilizes an updated receive burst detection algorithm that provides greater sensitivity and reliability than the original OpenBTS approach, which relied on energy detection for the initial stage of burst acquisition.

The limitation of the previous approach was that it was slow to adapt to highly transient power levels and false burst detection in challenging situations such as receiver saturation, which may occur in close range lab testing. The other issue was that a high degree of level tuning was often necessary to operate reliably.

The current receiver code addressed those limitations for improved performance in a wider variety of environments.

Low Phase Error Modulator

The default GSM downlink signal is configured for low distortion using a linearized GMSK modulator. The implementation is based on a two pulse Laurent approximation of continuous phase modulated (CPM) signals. The baseband output signal measures with very low phase error and is capable of passing industry spectrum mask requirements. Please note that actual performance will depend strongly on the particular device in use.

Theoretical details can be found in the report on GMSK. Octave / Matlab code for pulse generation is also available.

This option can be enabled or disabled at run time from the command line.

You can flash the FPGA data you just downloaded with the following command, setting type and other parameters accordingly to your hw. For instance for an Ettus B200:

uhd_image_loader --args="type=b200"

The uhd_image_loader claims it can update the firmware too, but at least on some versions it does nothing when asked to update firmware. If you see no output of firwmare being flashed, you can use this other command line to flash the firmware, adapting it to the firmware file of your HW:

Intel SSE support is automatically detected on Intel x86 platforms. No user intervention is necessary. The general configuration defaults to the low phase error modulator. Atom users may wish to use the low-CPU utilization modulator, which can be later enabled from the command line at runtime.

Many popular ARM development boards fall under this category including BeagleBoard, PandaBoard, and Ettus E100 USRP. This option will disable the low phase error modulator, which can be re-enabled at runtime. NEON support must be manually enabled.

$ ./configure --with-neon

ARM Platforms with NEON-VFPv4

Currently very few development platforms support this instruction set, which is seen mainly in high end smartphones and tablets. Available development boards are ArndaleBoard and ODROID-XU. This option will disable the low phase error modulator, which can be re-enabled at runtime. NEON-VFPv4 support must be manually enabled.

$ ./configure --with-neon-vfpv4

ARM Platforms without NEON

This configuration mainly targets the Raspberry Pi. ARM platforms without NEON vector units are almost always very slow processors, and generally not very suitable for running OsmoTRX. Running OsmoTRX on a Raspberry Pi, however, is possible along with limited TCH (voice) channel support. Currently this configuration requires minor code changes.

Coming soon...

Choosing your target device

Different SDR boards are managed using different software or libraries, usually provided by the vendor. As a result, different osmo-trx binaries can be built based on which device one targets. For instance, if support for LimeSDR is required, one must use the osmo-trx-lms binary, whereas if a UHD device is targeted, osmo-trx-uhd must be used, and so on. Build of different @osmo-trx binaries is controlled at configure time:

Normally simply start osmo-trx-uhd or similar, based on your target device. You only need to remember to pass a suitable config file.OsmoTRX can be configured with a variety of options. You can find examples for several different devices under doc/examples of osmo-trx.git directory.See section "OsmoTRX user manuals and documentation" where you can find links to the VTY reference.

OsmoTRX is fully compatible with OpenBTS for voice and SMS services. Due to differences in handing of GPRS, OsmoTRX does not support GPRS when used with OpenBTS, however, GPRS with the Osmocom stack is supported.

The OsmoTRX transceiver should be started before running OpenBTS. No symbolic link to './transceiver' should exist in the OpenBTS directory. This prevents OpenBTS from starting its own transceiver instance.

OsmoTRX is currently maintained by Tom Tsou and Alexander Chemeris among others. The code is derived from the OpenBTS project, which was originally developed by David Burgess and Harvind Samra at Range Networks.